This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, to a suitable substrate using an energy beam). Microlithography is a key technology used in the manufacture of microelectronic devices (e.g., semiconductor integrated circuits), displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, the invention pertains to charged-particle-beam (CPB) microlithography apparatus and methods exhibiting reduced deflection aberrations, and to methods for adjusting deflection aberrations to reduce them.
Charged-particle-beam (CPB) microlithography apparatus as developed over the past several years generally are grouped as follows: (1) spot-beam exposure systems, (2) variable-shaped beam exposure systems, and (3) block-exposure systems. In terms of resolution, each of these types of CPB exposure systems offers prospects of vastly superior performance than the previous batch-transfer optical microlithography systems (using visible or ultraviolet light). Unfortunately, each of these three types of exposure systems has poor throughput. The spot-beam exposure systems and variable-shaped beam systems exhibit especially poor throughput because they perform exposures by tracing the pattern using an extremely narrow beam typically having a square transverse profile (spot diameter). Block-exposure systems were developed to improve throughput by grouping groups of pattern features into uniformly shaped blocks each containing a partial pattern, and batch-exposing the blocks. However, because the number of pattern elements that can be placed on a reticle is limited, throughput was not improved as much as expected.
The development of xe2x80x9cdivided-reticlexe2x80x9d CPB microlithography methods and apparatus offered prospects of improved throughput. In these methods and apparatus, the reticle is divided or segmented into a large number of pattern portions that individually and sequentially are projection-exposed onto the substrate. A conventional divided-reticle apparatus is depicted in FIGS. 6 and 7.
FIG. 6 depicts a wafer that has been exposed with multiple xe2x80x9cdiesxe2x80x9d each corresponding to an individual xe2x80x9cchip.xe2x80x9d As can be seen, each die is divided into multiple xe2x80x9cstripes,xe2x80x9d and each stripe is divided into multiple xe2x80x9csubfields.xe2x80x9d The reticle (not shown) defining the pattern transferred to each die is divided similarly. Each subfield on the reticle is exposed and thus transferred to the wafer individually.
A typical divided-reticle microlithographic exposure of a die pattern is depicted in FIG. 7. For exposure, the reticle is mounted on a movable reticle stage and the wafer is mounted on a movable wafer stage (neither stage is shown). From FIG. 7, it can be seen that the pattern as defined on the reticle is larger (by a predetermined magnification ratio) than the pattern as transferred onto the wafer. In other words, as the pattern is transferred from the reticle to the wafer, the pattern is xe2x80x9cdemagnifiedxe2x80x9d by a xe2x80x9cdemagnification ratioxe2x80x9d which is the reciprocal of the magnification ratio. The demagnification ratio is a characteristic of the projection lens that is used to make the exposure. In view of the demagnification, during exposure the reticle stage and wafer stage are moved so that the centers of corresponding stripes on the reticle and wafer, respectively, travel at respective constant velocities that are related to each other by the demagnification ratio. As each subfield is exposed, it is illuminated by a CPB xe2x80x9cillumination beam.xe2x80x9d Portions of the illumination beam passing through the illuminated subfield become an xe2x80x9cimaging beamxe2x80x9d that is projected onto a wafer coated with a suitable resist. Projection is performed by passing the imaging beam through a projection-optical system.
Exposure normally is performed stripe-by-stripe and, within a stripe, subfield-by-subfield. During sequential exposure of the subfields in a stripe, the CPB illumination beam is deflected in a direction roughly perpendicular to the direction of travel of the reticle stage, thereby sequentially illuminating the subfields in a row within the stripe. The corresponding imaging beam is deflected similarly to place the subfield images properly on the wafer surface. As exposure of each row is completed, exposure progresses to the next row in the stripe with a concurrent reversal in the sweep direction of the beam. Exposure continues in this xe2x80x9crasterxe2x80x9d (switch-back) manner until all rows of subfields in the stripe are exposed. Exposure then progresses to the next stripe in the pattern. By sweeping the beam sequentially back and forth in a raster manner as shown in FIG. 7, throughput is improved compared to a scheme in which the beam is swept only in one direction across the rows of subfields.
In divided-reticle microlithography, since all the pattern elements in each of the subfield regions are exposed in a respective shot, and all the pattern elements to be transferred are defined on a single reticle, throughput can be improved markedly compared with other conventional CPB microlithography apparatus and methods.
In most reticles as used for divided-reticle CPB microlithography each subfield is surrounded by struts that strengthen the reticle (and separate the subfields one from another). Reticles used in optical microlithography typically do not have struts. Partly as a result of the struts, the illumination beam must be deflected and sized accurately to illuminate only the desired subfield at a given instant in time.
To increase throughput further in a divided-reticle microlithography apparatus, it is necessary to decrease the time during which the wafer and reticle are moving (to expose the next subfield) and to reduce the number of switch-backs. It also is necessary to minimize the overhead time consumed in starting and stopping the stages (to expose each subfield). One way in which this can be achieved is by expanding the beam-deflection range as much as possible. Unfortunately, as the magnitude of lateral deflection of a charged particle beam increases, deflection aberrations increase or arise. This is because, in being deflected a greater distance laterally, the beam passes through regions of the projection-optical system that are farther off-axis. Deflection aberrations are problematic because they cause blur and distortion of the image being exposed onto the wafer surface. Deflection aberrations can be reduced by adjusting the induction current of the deflectors used to deflect the beam and by configuring the deflection trajectory in a manner that reduces deflection aberrations. Although these remedies are useful to a limited extent, actual cancellation of deflection aberrations is desired through the use of corrective optical components such as stigmators (astigmatism compensators) and the like.
A stigmator conventionally is constructed by superimposing two quadrupole magnetic poles that are shifted 45xc2x0 from one another about the optical axis. The respective magnitudes of electrical current supplied to each quadrupole can be adjusted separately. Hence, respective magnetic-field components can be generated that are proportional to cos[2xcex8], where the field distribution is expressed in a cylindrical coordinate system (z,r,xcex8) in which xcex8 is the rotational angle around the optical axis. With such a scheme, aberrations proportional to the aperture angle of the illumination beam and aberrations proportional to the size of the illumination-beam subfield can be reduced or eliminated. Aberrations proportional to the illumination-beam aperture angle include, e.g., deflection astigmatism. Aberrations proportional to the size of the illumination-beam subfield include, e.g., deflection astigmatic distortion.
However, whenever the lateral beam-deflection distance is great, non-linear aberrations relative to the aperture angle and subfield size that are generated by even higher-order magnetic-field distributions become sufficiently large as to be no longer negligible and hence become problematic. Field components that are proportional to cos[3xcex8] and sin[3xcex8] primarily originate in manufacturing errors in the deflection coils and produce so-called four-fold aberrations. Four-fold aberrations originating in the 3xcex8 magnetic-field component can be categorized into the following six types according to their parameter dependence: (1) deflection distortion, dependent only on the beam-deflection position (can be eliminated by adjusting the deflection current in the deflector); (2) deflection astigmatism, proportional to the aperture angle of the illumination beam (can be eliminated using a stigmator); (3) deflection astigmatic distortion, proportional to the subfield size (can be eliminated using a stigmator); (4) deflection coma, which is proportional to the square of the aperture angle of the illumination beam; (5) deflection hybrid astigmatism, which is proportional to the aperture angle of the illumination beam and the subfield size; and (6) non-linear distortion, which is proportional to the square of the subfield size. Of these six types, deflection coma, deflection hybrid astigmatism, and non-linear distortion cannot be eliminated using a deflector or stigmator.
Use of a device that generates a magnetic-field component proportional to cos[3xcex8] has been considered for possible elimination of deflection coma, deflection hybrid astigmatism, and non-linear distortion. This was attempted by overlaying two hexapole magnetic poles, shifted 30xc2x0 from each another about the optical axis. As with a stigmator, four-fold aberrations could be eliminated by adjusting the magnitudes of the respective fields generated by the hexapole magnetic poles so as to generate field components proportional to cos[3xcex8] in a selective manner. Such a device is referred to herein as a 3xcex8-field four-fold-aberration compensator.
Whereas non-linear aberrations can be reduced or eliminated by changing the settings of a four-fold-aberration compensator, the values of linear aberrations in the overall optical system also are changed because a four-fold-aberration compensator itself produces linear aberrations. Hence, a correction procedure such as that shown in FIG. 8 typically is used conventionally in optical systems having both an astigmatism compensator (stigmator) and a four-fold-aberration compensator.
Specifically, in step S1, deflection astigmatism and deflection hybrid astigmatic distortion are eliminated by adjusting the stigmator. Next, in step S2, the four-fold aberrations are eliminated by adjusting the four-fold-aberration compensator. Then, in step S3, a determination is made of whether the aberrations in the system that can be eliminated exceed an acceptable range. If the aberrations are within the range, then adjustment is completed. If they are outside the range, then the procedure returns to step S1, and steps S1 and S2 are repeated.
In the conventional protocol shown in FIG. 8, linear aberrations that reappear in step S2 are usually not much smaller than the linear aberrations that supposedly were eliminated in step S1. This is a problem because it slows the process of zeroing in on the aberrations in the system that can be eliminated and increases the number of times that steps S1 and S2 must be repeated. This problem is not unique to electron-beam exposure apparatus; it also appears in CPB exposure apparatus in general.
In view of the shortcomings of conventional apparatus and methods as summarized above, an object of the invention is to provide charged-particle-beam (CPB) exposure apparatus, and methods for adjusting them, allowing reduced numbers of times that adjustments of compensators (that correct aberrations that can be eliminated) must be repeated. Hence, deflection aberrations are reduced, yielding improved methods for manufacturing microelectronic devices.
According to a first aspect of the invention, devices are provided for correcting aberrations in charged-particle-beam (CPB) microlithography apparatus. An embodiment of such a device is used in conjunction with a projection-optical system situated and configured to direct the imaging beam to the substrate so as to form an image on the substrate. The device includes at least one magnetic-field-generating device situated in the projection-optical system and configured to generate a magnetic-field component proportional to cos[5xcex8].
It has been discovered that fluctuations in linear aberrations (arising whenever non-linear four-fold aberrations are eliminated or reduced by generating a magnetic-field component proportional to cos[5xcex8]) are smaller than the fluctuations in linear aberrations (arising whenever non-linear four-fold aberrations are eliminated or reduced by generating a magnetic-field component proportional to cos[3xcex8]). Hence, a device according to the invention is termed a xe2x80x9c5xcex8-field four-fold aberration compensator.xe2x80x9d The apparatus can include at least one stigmator.
Desirably, the 5xcex8-field four-fold-aberration compensator comprises multiple (at least two) 10-pole magnetic poles. With such a configuration, two 10-pole sets of magnetic poles are displaced rotationally from each other such that adjacent magnetic poles are 18xc2x0 apart from each other about an optical axis. Of course, three or more 10-pole sets of magnetic poles can be used, with the magnetic poles being equi-angularly displaced from each other.
Another aspect of the invention is directed to CPB-microlithography methods in which an illumination beam is illuminated onto a region of a reticle to form an imaging beam, and the imaging beam is passed through a projection-optical system to form a corresponding image on a sensitive substrate. With respect to such methods, methods according to the invention are directed to controlling four-fold aberrations of the imaging beam. In an embodiment of such a method, the projection-optical system (imaging-optical system) is provided with at least one magnetic-field-generating device that generates a magnetic-field component proportional to cos[5xcex8]. An output of the magnetic-field-generating device is adjusted so as to produce the magnetic-field component proportional to cos[5xcex8].
As will be explained in detail in the detailed description, according to this method, fluctuations in linear aberrations (arising whenever non-linear four-fold aberrations are eliminated or reduced by generating a magnetic-field component proportional to cos[5xcex8]) are smaller than fluctuations in linear aberrations (arising whenever non-linear four-fold aberrations are eliminated or reduced by generating a field component proportional to cos[3xcex8]). Hence, this method allows elimination of non-linear four-fold aberrations without generating the large linear aberrations that were produced in conventional methods. This method also allows more rapid adjustments of aberration correction involving an iterative reduction in linear aberrations and non-linear four-fold aberrations.
In the foregoing method, a stigmator can be provided. Respective outputs of the stigmator and the magnetic-field-generating device are adjusted to maintain one or more of deflection astigmatism, deflection hybrid astigmatic distortion, and four-fold aberrations within respective pre-set tolerances. Since fluctuations in linear aberrations (arising whenever nonlinear four-fold aberrations are eliminated or reduced by generating a magnetic-field component proportional to cos[5xcex8]) are smaller than the fluctuations in linear aberrations (arising whenever non-linear four-fold aberrations are eliminated or reduced by generating a field component proportional to cos[3xcex8]), by alternately adjusting the outputs summarized above, the number of iterations needed to make the adjustments to within tolerances is reduced. Also, the various aberrations can be kept smaller compared to conventional methods in which outputs (of a stigmator and of a device for generating a field component proportional to cos[3xcex8]) are adjusted in an alternating manner.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.